Implantable knee prostheses for diseased and/or damaged knees typically include three components, namely a femoral component, a tibial component and a meniscal component. The femoral component may also include a patellar surface. The patella bone may be resurfaced with a prosthesis or left unresurfaced. The prosthesis components are generally configured to restore or emulate as much of the natural motion of the knee joint as possible. The selection of the particular prosthesis components is usually dictated by the condition of the patient's knee. For instance, the condition of the distal end of the femur and proximal end of the tibia, as well as the patency of the surrounding ligaments and soft tissue can affect the form of the joint prosthesis.
Generally, a total knee joint replacement includes a tibial component having a platform portion which replaces the entire superior surface of the tibial plateau and substitutes for the tibial condylar surfaces. The femoral component also includes laterally-spaced condylar portions joined by an inter-condylar bridge and a patellar surface. The inter-condylar bridge is in the form of a groove. The mating surfaces are smoothly curved in the anterior-posterior (AP) direction to generally match the lateral profile of the natural femoral and tibial condyles, and to ultimately replicate the normal joint movement.
A patella typically includes a ridge on its posterior face that separates the medial and lateral facets of the patella. The ridge rides within the inter-condylar groove as the knee is flexed so that the patella tracks along the groove. However, the movement of the various knee components is very complicated, and even shaped replacement components may not perform as well as a healthy, natural patella. For example, fluoroscopic studies have shown that when a leg is flexed to create a ninety degree angle, a natural patella will exhibit a 7 degree tilt. A common replacement patella, however, exhibits a 25 degree tilt.
There are two types of patellar replacement components that are commonly used. One type, which is the most commonly used, is constructed entirely of polyethylene. The other type is made from multiple subcomponents. In a multiple subcomponent patellar replacement component, there will typically be an articulating subcomponent made from polyethylene that is attached to a base. The base is implanted into the resected patella and is constructed from metal. The base in this type of patellar replacement component is of a uniform width while.
Initial implantation of a patellar replacement component is accomplished after resection of the natural patella. Resection is typically performed by first attaching a cutting guide to the patellar bone and cutting the patella along the angle defined by the quadriceps tendon and the patellar tendon as directed by the guide. The replacement component is then attached to the resected patella using a base that has a uniform depth. A problem with this approach is that patellae are not identical from individual to individual, or even from knee to knee in a particular individual. Thus, while the angle defined by quadriceps tendon and the patellar tendon is generally acceptable, it may not be the optimal angle for the particular patella being resected. Thus, when the base is attached, the patellar replacement component is aligned differently with respect to the femur as compared with the natural patella.
Replacement patellar components may not perform as well as natural patellae and can present a variety of undesired results. The misalignment can result in subluxation or dislocation of the patella. This may result in increased cartilage wear and knee pain. Of course, misalignment also results in increased wear of the various components. Another common problem is “patellar clunk syndrome”. In a knee exhibiting this syndrome, the patella “snags” as the knee is flexed and then extended resulting in patella displacement. However, as the pressure on patella increases, the snag is overcome, and the patella forcefully moves past the snag, typically impacting another surface. This impact causes pain and may even result in an audible “clunk”.
One approach to solving the maltracking problem experienced with replacement patellar components is to use shaped bone contacting surfaces including dome shapes and “saddle” shapes. Saddle shapes resemble hyperbolic paraboloids with two high ends and a low middle. The high ends are designed to track the inter-condylar groove.
Performance of shaped bone contacting surfaces is enhanced by providing for some relative motion between the pieces of the patellar replacement component. Enhanced performance is realized because of the principle that an object being acted upon by an external force will naturally turn until the largest possible area of the object is exposed to such a force. Accordingly, as, for example, a saddle shaped patellar replacement component begins to rotate up and out of the inter-condylar groove, if relative motion between the bone contacting surface and the rest of the replacement component is allowed, the bone contacting surface will not rotate, thus keeping both of the higher ends within the inter-condylar groove. As the force acting upon the patellar replacement component subsides, the bone contacting surface merely settles within the inter-condylar groove.
However, there are a limited number of known modalities for providing a patellar replacement component with some degree of freedom to either translate or rotate. Some replacement components combine some translational capability with rotational capability. However, these replacement parts are very limited in the degrees of rotation and/or translation provided. It would be beneficial to provide a variety of combinations of translational and/or rotational movement between pieces of a patellar replacement component.
Moreover, in order to allow relative motion between the various parts of a patellar replacement component, some assembly is needed. Of course, some assembly of patellar replacement component is commonly performed in order to use different types of material in the component. More specifically, the base is typically made of a material such as cobalt chrome alloys, although other materials such as ceramics, carbon based alloys or titanium alloys may be used. The bone contacting surface is made of polyethylene or ceramics. Nonetheless, assembly of moving parts presents additional design considerations. For example, the fit of the parts must be loose enough so that movement is not hindered, but the assembled parts must nonetheless be securely joined.
Additionally, some allowance must be made for the replacement of parts that have worn out after implantation. For example, it is useful to merely replace the bone contacting surface of a patellar replacement component than to replace the entire patellar component. Such limited replacement, however, is normally done within an incision area. Thus, it would be beneficial if a patellar replacement component included parts that could easily be assembled within a constricted space while ensuring that they remain securely assembled.
What is needed therefore is a patellar replacement component which overcomes one or more of the above-mentioned disadvantages.